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After is defined as both a preposition and a conjunction, as is before.

In a sentence that uses after or before, how can one determine whether it is functioning as a preposition or a conjunction?

Take, for example, these sentences:

  1. Before/After breakfast, Mac had a headache.
  2. Before/After eating breakfast, Mac had a headache.
  3. Before/After he ate breakfast, Mac had a headache.

What is the grammatical function of before/after in each of these sentences? How did you determine whether after/before was a preposition or a conjunction?

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Here are your examples with "before":

Before breakfast, Mac had a headache.

Preposition. Reason: it takes a noun, breakfast, as its object. "Before breakfast" is a prepositional phrase, which functions as an adverb, modifying "Mac had a headache."

Before eating breakfast, Mac had a headache.

Preposition. Reason: it introduces a prepositional phrase, ending in a noun, as in the previous sentence. "Eating" here is a gerund, serving as a noun. Notice that you could say "Eating breakfast starts your day off right." The verb in that sentence is starts, which is singular to agree with eating, the subject.

Before he ate breakfast, Mac had a headache.

Conjunction. Reason: it introduces a whole clause that has its own subject. "He ate breakfast" could be a whole sentence. "Before" converts it into a subordinate clause.

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    But it is not a different use of the preposition which creates the expectation, it is the inflection on the pronoun. We do not assign the verb to one class with I remember him helping and another with I remember he helped; why should we treat prepositions differently? – StoneyB Mar 6 '15 at 2:20
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    @StoneyB As always, there's more complexity. But I think these concepts are useful for ordinary people to understand for talking about sentences in ordinary situations--like teaching the language or fixing up a sentence. In practice, the traditional concepts are simple and intuitive, and they explain the most important things to know about grammar, like opening up a prepositional phrase vs. opening up a clause (in this example). Contemporary grammar is surely superior for linguistics, but from what I've seen so far, it's overwhelming and not useful for everyday, non-scholarly talk. – Ben Kovitz Mar 6 '15 at 2:42
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    Well, you get your simplification at the cost of complicating your terminology; and I suggest that you also lose the opportunity of emphasizing the critical matter of licensing, which we've seen come up over and over again here. I think, too, that our questioners are not well served by pedagogical techniques ('baby rules') suitable to primary and secondary school students; they're mostly adults, pretty sophisticated and literate, who are trying to master English for use in a university or grad school or professional setting. – StoneyB Mar 6 '15 at 2:58
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    ... But there's room for all sorts of approaches here. Indeed it is the particular strength of the SE format that questions get answers - good answers - suitable to every level of understanding and all sorts of needs. – StoneyB Mar 6 '15 at 3:02
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    @pyobum In this case that's wrong, really. It's very damaging to make learners distinguish between things that aren't significantly different in any way - escpecially when this means that learners don't understand why they are THE SAME. It means they won't understand the properties of the words they are talking about. Making students distinguish between the verb know when it takes a clause or a noun or is intransitive is a waste of learning time. Same is true of distinguishing whether a preposition is taking a finite clause, non-finite clause, noun or is intransitive. It's a waste ... – Araucaria Mar 6 '15 at 10:25
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Traditional grammar distinguished between different uses of words like before and after, calling them a) prepositions when they take phrases as their objects, and b) subordinating conjunctions when they introduce clauses. If you are taking tests, and your teacher makes the distinction, that is the basis for distinguishing them.

But there is no useful distinction here—the meaning doesn't change—so contemporary grammar calls them prepositions in both cases.

What you call the words doesn't matter at all, as long as you understand what they mean and how they mean it.

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    @pyobum The same way I explain that while can take a bare clause but not a that clause or a noun phrase, or that want takes marked infinitive clauses but not unmarked infinitive clauses or that clauses. Different words of the same class license different complements; that's just a fact of the language, and one that learners have to accept. – StoneyB Mar 6 '15 at 2:43
  • @pyobum How do you go about telling learners that different verbs take nouns, or interrogative clauses or predicative complements? I don't think it would help learners to split verbs into several new parts of speech to remember ;) – Araucaria Mar 6 '15 at 12:49
  • @pyobum In case you find it helpful, Oxford modern English grammar calls prepositions conjunctive prepositions when they take finite clauses :) – Araucaria Mar 6 '15 at 13:18
  • @pyobum Well you seem to want to split prepositions into different parts of speech because of what categories of word they take! Why not verbs too? At least that would be consistent! (I'm kind of joking - and kind of not. It's exactly why people realised that all prepositions were prepositions ...) – Araucaria Mar 7 '15 at 1:27
  • @pyobum Well, that's kind of the point, the grammar points to prepositions and subordinating conjunctions being the same part of speech, in the same way that the verb know is just a verb whether it takes a noun or a clause or nothing at all. These prepositions have the same syntactic function regardless of what comes after them. So it's exactly their context that demonstrates that prepositions are still prepositions, even if they aren't followed by a noun. But yes, we can agree to disagree :) – Araucaria Mar 7 '15 at 21:24

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